In modern high speed sheet processing machines such as xerographic systems, a sheet misfeed or multi-fed sheets can seriously impair the operation of the machine. Numerous devices of the type disclosed in U.S. Pat. No. 3,768,803 have been employed to minimize the possibility of misfeeds and multi-feeds.
To provide a constant normal force between the sheet being fed and the feed mechanism, it is common practice to employ a sheet elevator tray assembly having sheets stacked thereon, the feed mechanism being disposed above the stack for feeding the top sheet from the stack. As sheets are fed from the stack, the elevator is continuously shifted in an upward direction to maintain the top sheet in the stack adjacent the feed mechanism. This type of feeder is ordinarily used when a varying quantity of sheets may be loaded into the sheet tray since it has heretofore been very difficult to provide a dependable bottom sheet feed device due to the constantly varying weight of sheets in the tray which must be handled if a bottom sheet feed device is used. While it is more desirable and more convenient to use a bottom feed device since the sheet stack may be replenished without stopping machine operation and the complex elevator mechanism may be eliminated, the poor paper feeding capabilities of bottom feed devices has prevented common acceptance thereof. In U.S. Pat. Nos. 1,945,527 and 1,945,248 (Winkler), it is suggested that an air cushion stack levitation device be utilized to provide accurate, dependable bottom sheet feeding. U.S. Pat. No. 2,806,696 (Bishop) suggests the use of "riffle air" to help separate and lift the stack from the bottom sheets. However, in attempting to build a bottom sheet feeder in accordance with the teachings of the aforementioned patents it is soon apparent that the construction of a bottom air floatation feeder is not as simple as the patents would suggest.
It is therefore an object of the present invention to provide an improved air floatation stacking tray adapted for supporting a stack of sheets therein to enable consistent, accurate feeding of single sheets from the bottom of the stack with minimal possibilities of multi-feeds and misfeeds.